It is known from inter alia, EP-A-0 295 117 and U.S. Pat. No. 5,232,940, that certain N-phenylpyrazole compounds are useful for the control of arthropod, plant nematode, helminth and protozoan pests. These compounds include N-phenylpyrazoles having an optionally substituted amino group attached to the 5-position. Such substituted amino groups include amino substituted by one or two groups selected from alkyl and alkanoyl. Compounds of interest include those having a cyano group attached to the 3-position and a group RS(O)n attached to the 4-position, R being selected from alkyl and haloalkyl and n being 0, 1 or 2.
Among the compounds in the above-mentioned publications is listed 5-amino-3-cyano-1-(2,6-dichloro-4-trifluoromethylphenyl)-4-trifluoromethyl sulfinylpyrazole, which is depicted as the following formula:

This compound is currently used commercially to control pests in, for example, agriculture, public health and animal health, and is known as fipronil. Fipronil is a broad spectrum insecticide, toxic by contact and ingestion. It is used to control multiple species of thrips on a broad range of crops by foliar, soil or seed treatment, control of corn rootworm, wireworms and termites by soil treatment in maize and control of boll weevil and plant bugs on cotton, diamond-back moth on crucifers, Colorado potato beetle on potatoes by foliar application. It is also widely used in household pest control including roach and ant control and as a termiticide, as well as for treatment of household pets or other animals.
There is an urgent and unmet need in the art for efficient methods for the preparation and purification of fipronil, which are simple and can be used on a large scale for industrial manufacture, and which produce highly pure product that can be safely utilized.